/*
 * Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
 */
package com.opensource.dbhelp.dbutils.handlers;

import java.sql.ResultSet;
import java.sql.SQLException;

import com.opensource.dbhelp.dbutils.ResultSetHandler;

/**
 * <code>ResultSetHandler</code> implementation that converts one <code>ResultSet</code> column into an Object. This class is thread safe.
 *
 * @param <T>
 *            The type of the scalar
 * @see com.opensource.dbhelp.dbutils.ResultSetHandler
 */
public class ScalarHandler<T> implements ResultSetHandler<T> {

	/**
	 * The column number to retrieve.
	 */
	private final int columnIndex;

	/**
	 * The column name to retrieve. Either columnName or columnIndex will be used but never both.
	 */
	private final String columnName;

	/**
	 * Creates a new instance of ScalarHandler. The first column will be returned from <code>handle()</code>.
	 */
	public ScalarHandler() {
		this(1, null);
	}

	/**
	 * Creates a new instance of ScalarHandler.
	 *
	 * @param columnIndex
	 *            The index of the column to retrieve from the <code>ResultSet</code>.
	 */
	public ScalarHandler(int columnIndex) {
		this(columnIndex, null);
	}

	/**
	 * Creates a new instance of ScalarHandler.
	 *
	 * @param columnName
	 *            The name of the column to retrieve from the <code>ResultSet</code>.
	 */
	public ScalarHandler(String columnName) {
		this(1, columnName);
	}

	/**
	 * Helper constructor
	 * 
	 * @param columnIndex
	 *            The index of the column to retrieve from the <code>ResultSet</code>.
	 * @param columnName
	 *            The name of the column to retrieve from the <code>ResultSet</code>.
	 */
	private ScalarHandler(int columnIndex, String columnName) {
		this.columnIndex = columnIndex;
		this.columnName = columnName;
	}

	/**
	 * Returns one <code>ResultSet</code> column as an object via the <code>ResultSet.getObject()</code> method that performs type conversions.
	 * 
	 * @param rs
	 *            <code>ResultSet</code> to process.
	 * @return The column or <code>null</code> if there are no rows in the <code>ResultSet</code>.
	 * @throws java.sql.SQLException
	 *             if a database access error occurs
	 * @throws ClassCastException
	 *             if the class datatype does not match the column type
	 * @see com.opensource.dbhelp.dbutils.ResultSetHandler#handle(java.sql.ResultSet)
	 */
	// We assume that the user has picked the correct type to match the column
	// so getObject will return the appropriate type and the cast will succeed.
	@SuppressWarnings("unchecked")
	@Override
	public T handle(ResultSet rs) throws SQLException {

		if (rs.next()) {
			if (this.columnName == null) {
				return (T) rs.getObject(this.columnIndex);
			} else {
				return (T) rs.getObject(this.columnName);
			}

		} else {
			return null;
		}
	}
}